Scientists have rejected the controversial theory that smoking protects against developing Alzheimer's disease and other forms of dementia.

Their conclusions are based on a huge study of more than 34,000 male UK doctors whose smoking habits were reviewed every six to 12 years since 1951.

Previous research had suggested that smoking may boost the functioning of neurotransmitters in the brain known to be damaged by Alzheimer's disease.

The public health message is clear: at the population level there is no protective effect of smoking in dementia

Carol Brayne, Institute of Public Health, Cambridge

But the new research found no evidence of any protective effect.

More than 24,000 of the doctors had died by the end of 1998. Dementia was mentioned on the death certificates of 483.

The prevalence of both Alzheimer's disease and other dementias was similar in both smokers and non-smokers.

If anything, the authors conclude, persistent smoking may increase rather than decrease the rate of onset of dementia.

They argue that the previous studies were flawed because they were too small, and had relied on information about smoking habits from people other than the sufferers themselves.

The research, published in the British Medical Journal, was led by the eminent cancer expert Professor Sir Richard Doll, of Oxford University.

He said: "We conclude that the inverse relation
between Alzheimer's disease and smoking reported in small retrospective studies was largely or wholly artefactual.

"Persistent smoking does not reduce the age specific onset rate of the disease, or of dementia in general to any substantial extent.

Smokng was thought to boost brain function

"If anything, persistent smoking may increase rather than decrease the onset rate of dementia, but any net effect on severe dementia cannot be large in
either direction."

In an accompanying editorial, Dr Carol Brayne, from the Institute of Public Health, Cambridge, writes that the protective effect of nicotine is biologically plausible.

But she adds that the effect is likely to be short-lived, and that in the long-term smoking is likely to increase the risk of dementia by damaging the blood vessels supplying essential nutrients to the brain.

"The public health message is clear: at the population level there is no protective effect of smoking in dementia," she concludes.